1. Field of the Invention
This invention relates to a method and an apparatus for agglomerating finely divided agglomerative materials in a rotating drum and more particularly to a method and an apparatus for agglomerating finely divided coal particles and finely divided particles of carbonaceous residue in a rotating drum to form carbonaceous agglomerates.
2. Description of the Prior Art
The process for making formcoke as described in U.S. Pat. Nos. 3,073,751; 3,401,089 and 3,562,783 includes introducing particulate bituminous coal and finely divided char (the solid carbonaceous residue of coal which has been distilled at a temperature of between 800.degree. F. and 1400.degree. F.) in a rotary retort. Depending on the type of coal employed and the ratio of coal to char, pitch may also be added as a binder and to increase the strength of the agglomerates formed in this process. Preferably, the particulate coal and finely divided char are heated to an elevated temperature before they are introduced into the rotary retort so that the constituents supply as sensible heat substantially all of the heat required to achieve the desired temperature for agglomerating the carbonaceous materials.
During the agglomeration process the retort is rotated to effect intimate mixing of the constituents and tumbling of the agglomerates as they are formed. As the constituents are mixed in the retort the coal particles are further heated to such extent that partial distillation of the coal particles occurs, evolving tar and forming a loosely coherent plastic sticky mass in the retort. Where a pitch binder is employed it further contributes to the agglomeration of the particulate material within the retort.
It is believed that the loosely coherent plastic mass formed in the rotary retort breaks up during tumbling into relatively fine plastic particles. Growth of the plastic particles is attained by a snowballing type of tumbling or rolling action on the upper exposed inclined surface of the plastic mass of particulate material in the retort. Repeated tumbling or rolling of the particles causes the continued growth of the plastic particles into agglomerates. The agglomerates continue to grow until the binder evolved by the coal particles and the pitch binder, if employed, loses its plasticity. Thereafter, the agglomerates rigidify and the growth process is stopped. The agglomerates recovered from the agglomerating retort are thereafter calcined at an elevated temperature between 1500.degree. F. and 1800.degree. F. and formcoke is obtained that has strength and abrasion resistance that is equal or superior to that of conventional blast furnace coke. One of the objectives of the above described formcoke process is to form closely sized agglomerates having a suitable size range as, for example, a size range of between 3/4 .times. 2 inches or a size range of between 1 .times. 3 inches. Oversized agglomerates, i.e. agglomerates having a size greater than the desired size, and undersized agglomerates, i.e. agglomerates having a size less than the desired size, may not be suitable for use in a conventional blast furnace or other conventional metallurgical processes.
It has been discovered that, in conventional sized retorts, agglomerates of a suitable size range can be obtained in shallow beds where the ratio of the absolute bed depth of the particulate material to the diameter of the retort is maintained below a critical value. The absolute bed depth designates the true dimensional depth of the bed occupied by the carbonaceous materials and is measured at the deepest point in the bed of carbonaceous material within the rotary retort. It has been found where the ratio of absolute bed depth to retort diameter is maintained below the critical ratio (shallow bed) substantially all of the agglomerates have a size less than 4 inches and a substantial portion of the agglomerates have a suitable size range. Where, however, the ratio of absolute bed depth to retort diameter is increased above the critical ratio to form a deep bed the agglomerate product formed has a substantial quantity of agglomerates with a size greater than 4 inches and a reduced quantity of agglomerates with the suitable size range.
From an economic standpoint it is desirable to use retorts having as large a diameter as possible and to maintain as deep a bed of carbonaceous material as possible in the rotating retort. With these conditions, however, it is also essential that the size range of the agglomerate product formed be within the suitable size range.
In U.S. Pat. Nos. 3,368,012 and 3,460,195 there is disclosed a rotary retort for agglomerating carbonaceous material in a deep bed in which the ratio of absolute bed depth to retort diameter may be increased above the aforementioned critical ratio and a substantial yield of agglomerates of a suitable size range is obtained. In accordance with the teaching of these patents, preheated particulate bituminous coal and finely divided char are agglomerated into a rotary retort that has a plurality of longitudinally extending rakes secured to the rotary retort inner cylindrical wall. Each of the rakes has a plurality of tines extending inwardly toward the center of the rotary retort and the tines have a length between one-fourth and one-third the diameter of the rotary retort. The tines on the rakes are spaced from each other a preselected distance to relieve the compaction pressure exerted on the bed of carbonaceous material and to control the size of the agglomerates formed in the retort. An agglomerate product having a suitable size range is obtained even when the ratio of absolute bed depth to retort diameter is increased substantially above the ratio previously considered the critical ratio to obtain an acceptable yield of agglomerates having a suitable size range.
During the agglomeration process the carbonaceous materials have a tendency to adhere as a sticky plastic mass to the inner wall of the rotary retort and to the rake tines. Separate apparatus is required to remove the accumulation of agglomerated carbonaceous materials adhering to the retort wall and to the rake tines. Because of the tine spacing difficulty is encountered in removing the deposits of carbonaceous material on the retort wall and on the rake tines. Further, a substantial amount of energy is required to rotate the retort while the apparatus, such as a fixed scraper device, removes the carbonaceous deposits from the retort inner wall and rake tines.
It is also known in the agglomeration of agglomerative materials that a smooth inner cylindrical wall of the rotary drum or retort is not the optimum surface for forming agglomerates. Various types of metallic lifters for the agglomerative material within the rotary drum have been proposed as, for example, the lifters disclosed in U.S. Pat. Nos. 3,124,338; 2,695,221; 2,926,079; 2,213,056 and 3,689,044. These lifters are not suitable, however, where the agglomerative material has a tendency to adhere to the inner wall of the drum. After a short period of time a layer of the agglomerative material is formed on the wall of the drum to a depth that is equal to or greater than the height of the metallic lifters. This layer of agglomerative material reduces and soon eliminates the effect of the metallic lifters.
U.S. Pat. No. 3,348,262 discloses a fixed scraper for controlling the thickness of the layer or coating of agglomerative material deposited on the inner surface of the rotating drum. The layer has a uniform thickness and a generally relatively smooth surface. U.S. Pat. No. 2,697,068 and 3,316,585 disclose rotary scrapers positioned within the rotary drum that are arranged to continuously remove agglomerative material from the inner wall of the drum and maintain a layer of the agglomerative material of a preselected uniform thickness on the wall of the drum. It is stated the layer of agglomerative material provides a surface that is superior to a smooth cylindrical wall.
U.S. Pat. No. 2,831,210 discloses a cutter bar positioned within the rotary drum adjacent the inner wall of the drum. The cutter bar has spaced teeth extending toward the drum inner wall. The cutter bar is arranged to reciprocate longitudinally relative to the drum wall and cut a series of allochiral left and right hand helical grooves in the layer of agglomerative material deposited on the drum inner wall. The rate of reciprocation of the cutter bar and the speed of rotation of the drum are controlled so that the helical grooves do not track each other on successive strokes of the cutter bar and thus provide a controlled roughness to the surface of the layer of agglomerative material on the drum inner wall. It is stated the roughened surface of the layer is superior to a smooth surface.
U.S. Pat. No. 2,778,056 discloses an agglomerating drum with a scraper positioned therein. The drum and scraper are arranged to rotate at preselected synchronous speeds with the scraper rotating in a direction opposite to the direction of drum rotation. The scraper disclosed is in the form of a "ribbon flight" conveyor and forms multiple convolutions of helical grooves in material adhering to the inner surface of the drum. The convolutions of the main portion of the scraper have the direction of a right handed thread so that the multiple convolutions formed in the surface of the material adhering to the inner surface of the drum are so inclined that loose material in the grooves tends to move back toward the inlet of the drum. Adjacent the ends of the drum the direction of thread rotation of the scraper is reversed to minimize spillage of the material fed into the drum and also accelerate the discharge of the agglomerates formed. The helical grooves formed in the material adhering to the inner surface of the drum extend generally circumferentially around the inner surface of the drum so that the material within the drum tends to roll or slide downwardly within the grooves and be carried back towards the entrance of the drum. The ridges formed between the grooves because of their generally circumferential arrangement around the inner surface of the drum do not function as lifters to mix the material within the drum.
In a method and an apparatus known to applicants for agglomerating finely divided agglomerative material in which a limited number of generally longitudinally extending ridges and valleys can be formed in a layer of agglomerative material deposited on the inner wall of a drum. The ridges and valleys are formed by an elongated scraper device positioned adjacent the drum inner wall and arranged to rotate in a direction opposite to the direction of the drum. The height of the ridges formed by the method and apparatus disclosed in the above mentioned co-pending application are limited by the direction of rotation of the scraper device relative to the drum. For example, where the scraper has a pair of radially opposed blades extending longitudinally along the scraper tube and where a maximum height of the ridge is desired only four ridges can be formed around the periphery of the interior wall of the drum. As the number of ridges increase the height of the ridges decrease. Thus, limitations on ridge height and the number of ridges formed in the wall are present where the rotary scraper rotates in a direction opposite to the direction of drum rotation. Under certain agglomerative conditions it is highly desirable for optimum agglomerative conditions to have a plurality of ridges of a height greater than the height attainable by rotating the scraper in a direction opposite to the drum and further to provide a greater number of ridges on the periphery of the drum having the desired ridge height.
There is a need for a method and apparatus to provide elongated generally longitudinally extending ridges and valleys in a layer of agglomerative material deposited on the drum inner wall and to further provide a greater number of elongated ridges and elongated ridges having a greater height than is possible with the method and apparatus disclosed in the above named co-pending application so that the other agglomerative material fed into the drum is more effectively lifted and mixed.